Semiconductor crystals have been used on photon radiation detection from far infrared, infrared, visible, ultraviolet, X-ray, gamma Ray, and even energized particle detection for decades. Different radiation detection needs require different semiconductor material and special technology accordingly. In order to collect photon-induced signal carriers, the space-charge-layer of an electronic-field needs to be established inside the semiconductor detector crystal. This normally is achieved by fabricating a P-N junction or a proper surface barrier contact. Because of the extreme high penetration of high-energy gamma rays, the detector depletion-zone, or space-charge-layer, needs to be enlarged enough depending on the need of detection efficiency. The thickness of the depletion layer depends on the reverse bias voltage applied to the junction area and the doping level of the semiconductor crystal. The higher the reversed bias voltage applied, the thicker the depletion layer will be. At the same time, the lower the impurity concentration of the applied semiconductor crystal, the thicker the depletion layer. A gamma ray semiconductor detector normally is reverse biased by thousands of volts to deplete the entire crystal region. But too high bias makes the final application very difficult. So 5000V has been the maximum bias in industry practice for this application(1,2).
A common semiconductor device has a depletion layer thickness of only μm-scale, which makes the detection of high-energy gamma ray almost impossible. Because of this doping impurity concentration of the applied semiconductor crystal for gamma ray detection needs to be suppressed significantly. Lithium drift technique was invented over 4 decades ago on both germanium [Ge(Li)] and silicon [Si(Li)] for the purpose of X-ray and gamma ray detection applications(3). Lithium drift silicon detector [Si(Li)] is still being manufactured today. But, the Lithium drift germanium detector needs to be kept at 80K to keep the drifted lithium from drifting away from the compensated impurity. This makes the detector hard to be used, and transported in field. So Ge(Li) detector was substituted by High Purity Germanium (HPGe) detector as soon as the HPGe crystal was achieved(4,5). The first applied semiconductor detector structure for this application was a sandwich planar configuration of an intrinsic semiconductor detecting layer sandwiched between a P+ and a N+ contact layer(6). The net residual active impurities concentration of a semiconductor crystal for gamma ray photon detecting need to be purer than 5×1010 cm−3 level, which is about 105 times purer than the typical unintentionally doped semiconductor crystal. Germanium, silicon, CdTe, and CdZnTe are commonly used for gamma ray detecting. But germanium is still the only semiconductor material that has been purified enough to be used as a High Purity (HP) semiconductor gamma ray detector. Even an HPGe planar detector is typically made to deplete less than 20 mm thick. A 100% or 150% relative efficiency planer gamma ray detector, efficient at 1.33 MeV, needs the applied germanium crystal to deplete over 100 mm thick. So the planar types of germanium detector are typically for X-ray and low energy gamma ray applications of very low efficiency.
Coaxial geometry HPGe detectors were invented to achieve a large active volume, (high efficiency), gamma ray detector. Over 95% of the HPGe detectors are fabricated in standard closed-ended coaxial configuration every year. There are also a few other kinds of special HPGe detector configurations being manufactured. They are all very close to a typical planar, coaxial, or a combination of these two kinds of structures for particular applications, and are typically of very low efficiency used in X-ray or low energy gamma ray detection. Higher efficiency, typically higher than 10%, gamma ray detectors have been using the standard closed-ended bulletized coaxial detector configuration for decades.